This invention relates to a method and apparatus for making a transferable dye medium that is readily adapted for use in a thermal printer and, more particularly, to a method and apparatus for making such a transferable dye medium that is capable of being re-used several times without noticeable deterioration in the printed image formed therefrom.
A thermal printer capable of printing a "hard copy" of a video image is described in copending application Ser. No. 384,284 now Pat. No. 4,496,955. In that printer, a web of thermally transferable material is advanced between a thermal print head assembly and a record medium. Upon the selective application of heat to the web of thermally transferable material from the thermal print head assembly, the material evaporates from the web, is transferred to the record medium and condenses thereon to form a viewable image. To produce a full color image, the aforementioned web of thermally transferable material is provided with successive sections or areas having thermally transferable material of respectively different colors, such as cyan, magenta and yellow.
The thermally transferable material used in the aforementioned web is a sublimable dye. Various sublimable dyes having the desirable colors may be used in order to form a web of sequential sections or areas as aforesaid.
In one technique for making a web of thermally transferable material, a base layer, such as paper, is coated with suitable dye material. The dye material may be uniformly dispersed in a binder, such as cellulose, and applied as a liquid to the base layer. Thereafter, the dye material is dried, and the resultant dye-coated base layer constitutes the transferable dye medium for use in a printer of the aforementioned type.
In the technique described above for making the transferable dye medium, the dye layer, that is, the layer of dye material which is coated upon the surface of the base layer, may not be uniform. Hence, the density of the dye in certain areas may be greater than in other areas. As a consequence, when this transferable dye medium is used to print a viewable image, an irregular image may result.
Another disadvantage of the transferable dye medium made in accordance with the aforementioned technique resides in the fact that the dye layer merely reposes on the surface of the base layer. Because of this, the bond between the base and dye layers is relatively weak such that most of the dye material is transferred to the record medium upon a single printing operation. Accordingly, the transferable dye medium cannot be re-used many times. This is not economically favorable because a large quantity of the dye medium would be needed in order to carry out several printing operations. Still further, there is a tendency for both the dye layer and the base layer to adhere to the record medium during printing. This may jam or otherwise damage the automatic thermal printing apparatus with which the dye medium is used.